The present invention concerns a vehicle drive assembly, including motor means and at least a continuously variable ratio transmission device which drives a driving wheel or a driving-axle of the vehicle, the transmission device including a planetary gear provided with an input shaft driven by the motor means and an output shaft coupled to the corresponding driving wheel or driving-axle, the planetary gear including three main elements, namely: a sun wheel, a planet wheel carrier provided with a plurality of planet wheels and a ring gear with an inner toothing meshed with the planet wheels, one of said three main elements being connected to the input shaft, another being connected to the output shaft and the third acting as a reaction element, the transmission device further including a positive displacement pump, driven by the reaction element of the planetary gear, and a hydraulic circuit passing through the pump and provided with at least an adjustable hydraulic valve.
The invention also concerns a continuously variable ratio transmission device, able to be used in such a drive assembly.
In a machine or vehicle driven by a motor, setting in motion the mass to be driven on starting imposes very unfavourable operating conditions on the motor for a few tens of seconds. This is true whether the motor is electric or an internal combustion engine. In other words, the torque available on starting is much lower than the nominal torque. Indeed, when there is a mechanical transmission between a motor and a element being driven, there is always a proportional relation between the respective speeds of the motor and the element being driven and the element being driven has to reach a certain rotational speed for the motor to be able to reach its nominal condition. Moreover, in the particular case of an electric motor, the current intensities are very high on starting, until the motor reaches a certain rotational speed. This causes heating and requires the use of complex current limiting devices.
Transmission devices of the type indicated hereinbefore allow these starting problems to be overcome, by appropriate control of the operating conditions of the pump driven by the planetary gear reaction element, this element preferably being the inner toothed ring gear. When the motor starts, it is possible to let the ring gear rotate initially without much torque. The speed of the output shaft driving the driving wheel can then remain at zero or very low for a short while. Since the motor encounters a low reaction torque it very quickly picks up speed. As soon as the motor rotates at a speed deemed sufficient to provide a high enough torque in good conditions, the reaction torque exerted by the ring gear is increased by braking or blocking the ring gear via the hydraulic circuit, which allows a strong enough motor torque to be transmitted through the planetary gear and the driving wheel to be started.
French Patent No. 853 542 discloses such a transmission device for motor vehicles and proposes using it as a gradual speed variator, transmitting the input power with variable and continuous output speeds between zero and a maximum speed. The inner toothed ring gear of the planetary gear further includes an outer toothing meshing on a loose additional pinion to form a gear pump. However, the drawbacks of this construction are that it is cumbersome and that the pressures in the pump produce a resulting radial force on the ring gear which is not zero, which means that the crown is prevented from being freely centred on the planet wheels. This may affect the working and lifetime of the planetary gear.
According to a variant suggested in German Patent Application No. 37 00 051, the ring gear forming the pump rotor may be provided with outer fins which slide radially and abut by sliding against an oval wall of the casing. However, such a pump would be fragile and would also require the ring gear to be very thick.
European Patent Application No. 838 606 discloses a transmission device of the type indicated in the preamble, wherein the ring gear of the planetary gear also constitutes the inner rotor of a xe2x80x9cgerotorxe2x80x9d or xe2x80x9cgear rotorxe2x80x9d type pump, this ring gear having outer teeth which mesh in the inner toothing of an eccentric rotating ring which surrounds it and which has one additional tooth. This construction has the drawback of being relatively expensive to manufacture, because of its double toothing. Moreover, it considerably increases the radial space requirement around the planetary gear.
According to the aforecited document, the continuously variable speed transmission device is inserted between the internal combustion engine and a vehicle driving-axle, the transmission output shaft being for example coupled to the driving-axle differential input pinion. In a variant, the vehicle includes to of these transmission devices, which are driven by the same motor and have a common output shaft which drives the driving-axle. The first device is used for driving forward, whereas the second does not transmit any torque, its pump rotating freely. Conversely, it is the second device which is used for driving in reverse, while the first rotates freely.
However, the authors of the present invention have found that the use of two or more transmission devices of the type described hereinbefore, for driving respectively two or more driving wheels of a vehicle, advantageously allows a hydraulic differential to be achieved between the wheels owing to an interconnection of the respective hydraulic circuits of the transmission devices.
More particularly, a first aspect of the invention concerns a vehicle drive assembly, including motor means, at least a pair of continuously variable ratio transmission devices which each drive a driving wheel or a driving-axle of the vehicle, and means for regulating the transmission devices, each transmission device including a planetary gear provided with an input shaft coupled to motor means and an output shaft coupled to the corresponding driving wheel or driving-axle, the planetary gear including three main elements, namely: a sun wheel, a planet wheel carrier provided with a plurality of planet wheels and a ring gear having an inner toothing meshed with the planet wheels, one of said three main elements being connected to the input shaft, another being connected to the output shaft and the third acting as a reaction element, each transmission device further including a positive displacement vacuum pump, driven by the reaction element of the planetary gear, and a hydraulic circuit passing through the pump and provided with at least an adjustable hydraulic valve, the regulating means being arranged to control said hydraulic valves to adjust the speed and/or the torque of the output shaft of each planetary gear, two points located on either side of the pump in the hydraulic circuit of each transmission device being connected to the corresponding points of the other transmission device by connecting hydraulic lines.
It will be noted that the transmission devices of such an assembly can be driven either by a common motor, which is preferable in the case of a heat engine, or by individual motors, for example electric motors in the case of an electric or hybrid-powered vehicle.
The invention also concerns a transmission device which can be used in such a drive assembly including a planetary gear provided with an input shaft coupled to motor means and an output shaft, a positive displacement pump incorporated in the planetary gear and associated with a hydraulic circuit provided with at least an adjustable hydraulic valve, and means for controlling said valve to adjust the speed and/or torque of the output shaft, the planetary gear being formed of a sun wheel, a carrier provided with a plurality of planet wheels and a ring gear having an inner toothing meshed with the planet wheels, the input shaft being connected to the sun wheel and the output shaft being connected to the planet wheel carrier or vice versa, the pump including a casing and a rotor formed at least in part by said ring gear, characterised in that the ring gear has a non-circular outer surface against which at least two segments, sliding radially in the pump casing, abut, said segments delimiting two symmetrical chambers from each other between the rotor and the pump casing.
The main advantages of this arrangement are that it reduces the space requirement, complexity and manufacturing cost of the pump, in particular owing to the simplicity of its rotor. The pump can thus occupy the same casing as the planetary gear without any significant increase in the size and complexity of the casing.